A prior art search was conducted at the United States Patent and Trademark Office, and uncovered the following references pertaining to phase shifters: U.S. Pat. Nos. 3,436,691; 4,205,282; 4,450,419; 4,458,219; 4,471,330; 4,568,893; 4,586,047; 4,599,585; 4,612,520; 4,647,789; and 4,652,883.
None of the above references employs coplanar waveguide as does the present invention. Coplanar waveguide is a microwave transmission line geometry in which a dielectric substrate supports three coplanar conductors: a center conductor conveying the signal plus two conductive ground plane elements, positioned on either side of the center conductor and separated therefrom by two air gaps having substantially the same width. The present invention's novel use of coplanar waveguide has led to the production of devices offering the following advantages over the prior art: greater accuracy; greater repeatability; lower cost; smaller size; greater ease of handling because the dielectric substrate can be made thicker; greater ease of fabrication because all of the circuitry is on the same side of the dielectric substrate; and circuit elements which are less dispersive, thereby enabling a constant time delay. Since the delay lines are made of top layer metals and are completely testable on wafer, performance of the devices can be verified before packaging.
The present invention also uses switching elements that are resonant at the center operating frequency. This technique, which is not suggested in the above references, offers the following advantages: minimization of the signal to the arm that is not switched; minimization of loading (which would cause unwanted reflections and increased VSWR) on the arm that is switched; and the ability to operate at a higher frequency while maintaining a small size.
Furthermore, most of the reference devices are true phase shifters, in which a constant phase shift rather than a constant time delay is imparted to the signal. This limits the device to a narrow band of frequencies. For one of the major applications of this invention--large, electrically-steerable antenna arrays--such constant phase shifters produce a beam angle that is inversely proportional to frequency, and therefore an unwanted pointing dispersion is the result. The present invention, on the other hand, imparts a constant time delay over a broader band than a phase shifter. The bandwidth is typically 10%. A constant group delay is imparted, which is equivalent to a phase shift that is linear with respect to frequency. For the aforesaid electrically-steerable antenna array application, this advantageously produces a beam angle that is constant for all frequencies of interest.
Gupta et al., "A 20 GHz 5-bit Phase Shift Transmit Module with 16 dB Gain", Proceedings of the 1984 IEEE GaAs IC Symposium, pp. 197-200, describes a time delay shifter using switched microstrip delay line and FET switches. This reference does not suggest the use of coplanar waveguide nor inductors used with the FETs to tune to resonance.
An additional reference, which gives a general discussion of coplanar waveguide, is Riaziat et al., "Coplanar Waveguides for MMICs", Microwave Journal, June 1987, pp. 125-131.